Not smiling for long: David Tennant in The End of Time. Photograph: BBC

This year's Christmas Doctor Who was never going to pause and sniff the mulled wine. There are stories that need completing, fireworks to set up; the End of Time means No End of Plot. We've had the Ood – I find them harder to take seriously since seeing the cover of Sense and Sensibility and Sea Monsters, but maybe that's just me – prophesying doom. We've had a ghostly Claire Bloom talking in riddles to Bernard Cribbins. We've had the Doctor facing up to death with enough fear in his eyes to make you almost forget about them filming his next incarnation. We've had one very sinister close-up of Timothy Dalton. Most of all, we've had John Simm's Master, brought most of the way back to life with unpredictable new powers and an appetite that has him more or less literally chewing the scenery. And that's before he turns every human on Earth into a copy of himself. It's been a rush.

The fun of a cliffhanger – and one of the distinctive virtues of Russell T Davies's Doctor Who has been the sense of how much fun he was having – comes from watching a writer strand their narrative cat up the tallest imaginable tree. This one's definitely up there. Has any previous Who story managed to erase humanity before the end of the first episode? Has humanity ever been destroyed in such a comic, family-Christmas-friendly manner? And just how much John Simm can we all take?

When the whole of this story is told, however – the second instalment comes on New Year's Day – it's the quieter, emotional parts near the beginning of tonight's episode that may prove to be most important. The irritating-going-on-worrying "Time Lord triumphant" of The Waters of Mars returns for a few minutes – just long enough to get in that nice, off-colour Elizabeth I gag – but it only takes four bangs on an oil drum to bring David Tennant's vulnerability to the surface. Cribbins, meanwhile, looks to have been sent out on the same tightrope previously traversed by Catherine Tate: becoming the tragic hero while remaining the comic relief. I reckon he's staying on so far.

More questions: What's Dalton up to? You don't wear a collar like that just to deliver a voiceover, even a really, really portentous one. (I'm hoping all that bombastic "did say" stuff turns out to have character justification.) And what's the logic behind who turns into the Master and who doesn't? We know why Donna doesn't – the same Time Lord streak that's giving her such a headache – but who gave Wilf a note excusing him from the human race?

And one other thing: how much do the imperatives of a two-parter detract from the traditional satisfactions of a Christmas Who? There are plenty of tinsel touches – two separate "best Christmas present" lines, the Queen's speech, a choir and a silver band, not to mention that turkey – but this didn't need to be a Christmas story in the way that previous Christmas Doctor Whos have. And it did need to leave you desperately wanting more.

So were you sated? Do you resent having to wait until New Year's Day for the pud? Or do you feel as if you just wolfed a whole turkey without the aid of cutlery?

• Phil Hogan's review of Christmas TV will appear on 27 December. Nancy Banks-Smith's review will appear on 28 December